1. Field of the Invention
The subject invention relates to apparatus for automatically fine-tuning a band-pass filter to the carrier frequency of a modulated signal applied to the filter.
2. Description of Prior Art
Persons skilled in the art to which the invention relates are at least constructively aware of systems, such as disclosed in U.S. Pat. Nos. 3,165,697 and 3,255,414, in which automatic fine tuning of a band-pass filter is achieved using an output signal of the filter to provide a signal which controls a tuning component of the filter. More specifically, U.S. Pat. No. 3,165,697 discloses a tuned cavity amplifier, the cavity of the amplifier being tunable with a plunger. In operation, the plunger is sinusoidally varied and a signal applied to the cavity amplifier results in the generation of an amplitude modulated signal. The amplitude modulation is detected and used in a feedback scheme to determine the position about which the plunger is sinusoidally varied. In the steady state, the position about which the plunger is varied substantially corresponds to the carrier frequency of the applied signal. U.S. Pat. No. 3,255,414 discloses a transmitter having an output tuned circuit including a controllable inductor. In operation, a first control winding of the inductor receives a low frequency signal (100 Hz) which periodically varies the resonant frequency of the circuit. With the circuit being varied, a signal applied to the tuned circuit results in the generation of an amplitude modulated signal. The phase of the modulation is dependent upon whether the frequency about which the turned circuit is dithered is higher or lower than the carrier frequency of the signal. A phase discriminator responds to the low frequency signal and the amplitude modulation to provide to a second control winding of the inductor a signal which tunes the circuit. In the steady state, the frequency about which the turned circuit is dithered corresponds to the frequency of the signal being transmitted.